A NEW TECHNIQUE TO MODEL PLASMOID MAGNETIC-FIELD SIGNATURES

A new fitting method has been developed and applied to the plasmoid ob servations of the GEOTAIL satellite. This method, which we call ''soft fitting'', is regarded as one kind of least square method, and can de termine both the fitting function and fitting parameters simultaneousl y. Its application to magnetic field observations by the GEOTAIL satel lite gives an average shape of plasmoid Bz bipolar structures. The met hod can also provide information about the velocity and spatial size o f plasmoids from the optimal fitting parameters. The result indicates that the velocity and size become larger as a plasmoid propagates tail ward.